FPO - buyers pulls out of sale: can this be done by accident?

It probably depends what they used eg Desktop view, mobile view, iPhone app or Android app. I don't have any FPOs I've been offered that are outstanding at the moment to be able to test unfortunately. 

From memory yes probably a couple of clicks at least. You can pretty easily test yourself if you want. Browse to a category jammed full of stuff (eg Mobile phone accessories) Sort by closing soon and add a bunch to your watchlist. 2 minutes later you will likely get a bunch of automated FPOs.

I would just let the buyer save face. Did they contact you straight away? If they did I would let it go. If you've had to chase them and then they came out with that excuse I might be harder on the feedback. Sometimes looking at the feedback they already have can give you good clues as well if it was a genuine mistake.

thanks for the feedback. just trying to see if it was a genuine mistake or not. if its more than a 1 click deal then yeah..